1. Field of the Invention
The present invention relates to an energy harvesting system and a power distribution method, and more particularly to an energy harvesting system using a simplified architecture and fewer switching elements to obtain high power conversion efficiency.
2. Description of the Prior Art
With reference to FIG. 10 in a conventional energy harvesting system, a low power consumption load such as a wireless sensor network system is suitably powered by ambient energy sources that generate small amount of electrical power. The ambient energy may result from ambient light, temperature difference, vibration and etc. An energy storage such as a secondary battery is preferably applied in the energy harvesting system to provide extra energy to the load under a heavy load situation to cover deficiency of the ambient energy. Contrary to the heavy load situation, the energy storage can obtain and store surplus energy from the ambient energy source when the load is under a light load situation. Therefore, power distribution among the ambient energy source, the energy storage and the load is a considerably important issue.
FIG. 11A shows a power flow chart of one type of energy harvesting system having two converters Z1 and Z2. The first converter Z1 converts the energy of the ambient energy source and stores the energy in an energy storage. The second converter Z2 converts the energy in the energy storage and outputs the converted energy to the load.
With reference to FIG. 11B, a two-stage conventional DC to DC energy harvesting system is proposed to implement the power flow of FIG. 11A. The two-stage conventional DC to DC energy harvesting system for converting the ambient energy includes a first converter Z1 and a second converter Z2 connected sequentially. The first converter Z1 converts energy of an ambient energy source such as a solar panel and stores the energy in an energy storage. The second converter Z2 further converts the energy in the energy storage and outputs the converted energy to the load. Since the energy harvesting system uses two converters Z1, Z2, the power conversion loss is high, and the conversion efficiency is usually low. If each converter Z1, Z2 adopts an inductor L1, L2, the large size of the inductors will result in a bulky energy harvesting system.
FIG. 12A shows a power flow chart of another type of energy harvesting system having three converters Z1, Z2, Z3. The first converter Z1 converts the energy of the ambient energy source and supplies the converted energy to the load based on the load demand. The second converter Z2 transfers the surplus energy of the ambient energy source to the energy storage. The third converter Z3 converts the energy of the energy storage and supplies the energy to the load to cover deficiency of the ambient energy.
With reference to FIG. 12B, a single inductor multi-input multi-output (SIMIMO) energy harvesting system is proposed to implement the power flow of FIG. 12A. The SIMIMO energy harvesting system mainly comprises five power transistors S1-S5 and a single inductor L. The energy harvesting system adopts the time-division multiplexing (TDM) control and provides three converters Z1-Z3. Each of the converters Z1-Z3 can be implemented by the elements along a corresponding path shown in FIG. 12B. Comparing to the two-stage energy harvesting system, the number of inductors has been reduced to one. However, the SIMIMO energy harvesting system instead requires more power transistors S1-S2, S4-S5 as inductor-sharing switches to determine the input/output components of the single inductor that will cause extra switching loss and lower the power conversion efficiency.